US 2010/0257862 A1 describes a principle of a known energy storage installation, in which use is made of a piston-type machine. From U.S. Pat. No. 5,436,508, it is furthermore known that, by means of energy storage installations for storing thermal energy, it is also possible to temporarily store overcapacities that arise during the use of wind energy for producing electrical current.
During the charging of the accumulator, such energy stores convert electrical energy into thermal energy and store the thermal energy. During the discharging process, the thermal energy is converted back into electrical energy.
Owing to the time period that has to be bridged by an energy store, that is to say the time over which energy is stored into and released from the energy store, and owing to the power that must be stored, correspondingly high demands are placed on the dimensions of thermal energy stores. Thermal energy stores can thus be very expensive in terms of purchase costs simply owing to the structural size. If the energy store is furthermore of complex design, or the actual energy storage medium is expensive in terms of production costs or cumbersome in terms of operation, the purchase and operating costs for a thermal energy store can quickly call into question the economic viability of the energy storage.
Owing to the often low thermal conductivity of the expensive storage materials, the heat exchanger surfaces must often be designed to be very large. The large number and the length of heat exchanger pipes can in this case lead to a significant rise in costs of the heat exchanger, which costs can no longer be compensated by an inexpensive storage material.
Until now, in order to replace large heat exchangers, heat exchangers have been designed on the basis of relatively inexpensive materials, primarily in the form of heat exchangers for a direct exchange of heat between the heat carrier, for example air, and the storage material, such as for example sand or gravel. The fluidized bed technique known in principle in the art has not hitherto been implemented on a scale that would be required for seasonal storage of excess renewable energy. A direct exchange of heat furthermore entails relatively complicated handling of the solid matter, which is not economical for a large store.
As heat carrier medium, use is made of a working gas, for example air. The working gas may in this case be conducted optionally in a closed or an open charging circuit or auxiliary circuit.
An open circuit always uses ambient air as working gas. Said ambient air is drawn in from the surroundings and is discharged into the environment again at the end of the process, such that the environment closes the open circuit. A closed circuit also permits the use of a working gas other than ambient air. Said working gas is conducted in the closed circuit. Since an expansion into the environment, with the ambient pressure and the ambient temperature simultaneously being adopted, is omitted, the working gas must, in the case of a closed circuit, be conducted through a heat exchanger which allows the heat of the working gas to be released to the environment. Since, in a closed circuit, use may also be made of dehumidified air or other working gases, it is possible to dispense with a multi-stage configuration of the compressor and a water separator. A disadvantage here is however the additional cost outlay for the purchase and operation of an additional heat exchanger downstream of the expansion turbine, or upstream of the compressor, for heating the working gas to working temperature for the compressor. During operation, this reduces the efficiency of the energy storage installation.
It may alternatively be provided that the charging circuit for the storage of the thermal energy in the heat accumulator is in the form of an open circuit, and that the compressor is constructed with two stages, wherein a water separator for the working gas is provided between the stages. Here, allowance is made for the fact that air moisture is contained in the ambient air. An expansion of the working gas in a single stage can have the effect that, owing to the intense cooling of the working gas to, for example, −100° C., the air moisture condenses and hereby damages the expansion turbine. In particular, icing can cause permanent damage to turbine blades. An expansion of the working gas in two steps however makes it possible for condensed water to be separated off, in a water separator downstream of the first stage, at for example 5° C., such that, during a further cooling of the working gas in the second turbine stage, said working gas has already been dehumidified, and formation of ice can be prevented or at least reduced. Disadvantages here, too, are however the increased cost outlay for the purchase of a multi-stage compressor and of a water separator. Also, during operation, the efficiency of a plant of said type is reduced.